Bug 34346: Show error message instead of 500 for duplicate tags

Bug 34346: Show error message instead of 500 for duplicate tags

This change shows a user-friendly error message instead of a 500 error
if adding a duplicate MARC tag to a MARC bibliographic framework.

Test plan:
0. Add patch and koha-plack --reload kohadev
1. Go to
http://localhost:8081/cgi-bin/koha/admin/marctagstructure.pl?searchfield=264&frameworkcode=
2. Click "New tag"
3. Enter "264" in "Tag:" field
4. Click "Save changes"
5. See error message on screen (instead of 500 error)

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Bug 34346: Tidy

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Slightly adjusted error message and added punctuation.

Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>